There has been a long-standing desire to ensure the ability of gas turbine engines to be operated with "lower" grade fuels. A "low" grade fuel is a fuel having comparatively low octane and/or a comparatively high viscosity. The viscosity of jet engine fuel will also vary with the ambient temperature and thus even the better grade fuels will become less viscous when subjected to extreme cold. The ability to employ lower grade fuels, for example fuels derived from oil shale, would provide insurance against the interruption of normal petroleum supplies. Further, and this is particularly true in the case of the military, the necessity of using lower grade fuel may be dictated by local availability and/or the need to use fuel which has been in storage for a considerable period of time.
The characteristics of a fuel, particularly a jet engine fuel, are particularly significant during starting when the fuel is delivered to the burners at a comparatively low pressure. Under such conditions, if the viscosity of the fuel is too great there will be insufficient atomization and at least a significant portion of the fuel will be injected into the burners in the form of droplets. Accordingly, a fuel/air mixture appropriate for ignition will not be created and the engine will not start. The fuel delivered to the burners will, of course, accumulate and if ignition is subsequently achieved, this excess fuel will burn resulting in engine over-temperature and possible damage.
In the past, there have principally been two approaches to achieving rapid ignition and thus successful starting of a gas turbine engine. Firstly, and most obviously, better grade fuels, i.e., fuels having a comparatively high octane and comparatively low viscosity, were selected for use. The second approach has been to operate the engine with an additional enrichment schedule wherein excess fuel is deliberately delivered to the burners in a start mode. The excess flow, for a given supply pressure, results in a greater pressure drop across the fuel nozzles and thus produces a higher degree of atomization. However, employment of an enrichment schedule carries the inherent risk that, because of the excess fuel, there will be a "hot" start which may cause engine damage.